The present invention relates to a process for preparing a superconducting wire, and more particularly to a process for preparing a superconducting wire by using a composite billet wherein a superconductor or a material convertible into a superconductor by a heat treatment is filled in each of holes of a body having a multiplicity of holes composed of built-up Cu base metal plates stuffed in a supporting container.
As the field of application of superconducting materials which can allow to flow a heavy current or can generate a strong magnetic field, there are mentioned, for example, (1) development of energy saving technology by utilization of superconducting materials in electrical power systems such as generator, power transmission cable and energy storage, (2) development of new energy such as nuclear fusion or MHD power generation, and (3) development of new technology utilizing high magnetic field such as high energy accelerator, linear motor car, electromagnetic thrust ship, magnetic separation and medical MRI. Development of excellent technology for superconducting wire material is indispensable to growth or advance of application technology for superconducting materials. There have hitherto been developed NbTi alloy superconducting wire for application under a magnetic field of not more than 8 T and 9 T and Nb.sub.3 Sn and V.sub.3 Ga compound superconducting wires for application under a magnetic field of not less than 8 T and 9 T. These superconducting wires have, for stabilization thereof, a structure that a multiplicity of superconducting filaments having a diameter of several tens of micrometers or less are embedded in a metal matrix having a low electrical resistivity such as Cu and the filaments are twisted. Such superconducting wires are called fine multifilamentary wire.
Practical application of the fine multifilamentary wires started with an alloy type material having a good processability. A process for preparing an NbTi wire is briefly explained below. The details thereof are described in various publications, for example, Superconductive Engineering (Revised Edition) published by Kabushiki Kaisha Ohm-She, page 74, (1988), and Zairyokagakushi, Vol. 20, page 80 (1983).
First, NbTi alloy is subjected to cold working to form a round rod. The round rod is inserted into a Cu tube, and the resultant is processed to reduce the cross-sectional area, typically drawn to a small size, to form a single core wire. The single core wire is then cut into pieces having an appropriate length and a plurality of these wire pieces are stuffed into a container made of Cu. Air in the container is evacuated, and a cover is welded to the container to seal it up, thus giving a composite billet. Thereafter the composite billet is repeatedly subjected to an extrusion processing and the sectional area reduction processing to give a composite wire. When it is desired to produce a superconducting wire having a heavy current capacity, a multiplicity of the thus obtained composite wires may be stuffed into a Cu tube and drawn to reduce the sectional area. In general, the critical current density of NbTi alloy wire is greatly increased by combination of a high reduction processing (rate of sectional area reduction of not less than 10.sup.4) and annealing (heat treatment temperature of 350.degree. to 450.degree. C.). Therefore, the composite wire is usually subjected to repeated high reduction processing-annealing, and further to twisting processing to give a fine multifilamentary wire.
With respect to a process for preparing compound superconducting wires, an explanation is given below. The compound superconducting materials have an excellent feature that the critical temperature (Tc) and the upper critical magnetic field (Bc.sub.2) both are fairly high as compared with the alloy superconducting materials. On the other hand, they have the disadvantage of being very fragile. Therefore, since the compound superconducting materials themselves have no processability, various proposals have been made with respect to a process for the production of fine multifilamentary wires of these materials. The processes industrially established at present are those utilizing a solid phase reaction. The principal processes are a bronze process, a tube process, an internal tin diffusion process, an external tin diffusion process and the like, as known, for example, from Superconductive Engineering (Revised Edition) published by Kabushiki Kaisha Ohm-Sha, page 74 (1988) and Zairyokagakushi, Vol. 20, page 82 (1983). A typical internal tin diffusion process will be briefly explained below with reference to Nb.sub.3 Sn, since V.sub.3 Ga is qualitatively equivalent to Nb.sub.3 Sn if V is substituted for Nb and Ga is substituted for Sn.
First, an Nb rod is inserted into a Cu tube and processed to reduce the area in section to a certain diameter thereby giving a single core wire. The single core wire is cut into pieces having an appropriate length. A multiplicity of these wire pieces are stuffed into a container made of Cu, while a Cu rod or a bundle of Cu wires is disposed in the center portion of the container. Air in the container is evacuated, and a cover is welded to the container to seal up it, thus giving a composite billet. Then, the composite billet is subjected to extrusion processing, and a hole is drilled mechanically in the Cu portion in the center of the composite billet. An Sn rod is inserted into the hole, and the composite billet is circumferentially covered with a tube made of Ta or Nb, and further with a Cu tube. The resultant is processed to reduce the cross-sectional area. If it is desired to produce a superconducting wire having a heavy current capacity, a multiplicity of the thus obtained composite wires may be further stuffed into a Cu tube and drawn to reduce the sectional area. After drawing the thus obtained composite wire to a final diameter and subjecting to a twisting processing, it is subjected to a heat treatment, whereby Sn is diffused into the surrounding Cu to convert Cu into a Cu-Sn compound or alloy, and further to react with the Nb filaments to convert the surface region or the entire of the Nb filaments into Nb.sub.3 Sn, thus giving an Nb.sub.3 Sn fine multifilamentary wire.
Like this, a process for the preparation of a fine multifilamentary wire is being industrially established with respect to both an alloy superconductor and a compound superconductor. Recently, a high field magnet of not less than 17 T is put to practical use by adding a third element to an Nb.sub.3 Sn compound.
However, these conventional processes have disadvantages. A step of stuffing a multiplicity of Cu rods or wires or Cu/NbTi or Cu/Nb single core wires into a Cu container to produce a composite billet is the most important in the production of superconducting wires. Since the shape or structure of the fine multifilamentary wire is almost determined by this step, it may be safely be said that the good and bad of the result of this step governs the superconducting characteristics of the obtained wires. In conventional processes explained above, the composite billet has been prepared by inserting from several tens to one thousand several hundreds of the single core wires cut to an appropriate length into a Cu tube by means of many hands. Therefore, it requires much labor and time in order to satisfy the processing accuracy such as linearity of the single core wire, thus resulting in increase of preparation cost. Further, the conventional processes have a limit in packing density of the single core wires.
Also, hereafter, in order to meet the demand for further improved high performance of superconducting wires, it would be an important subject to further increase the number of superconducting filaments and to make the filament thinner. For achieving this subject, it is necessary to increase the number of single core wires packed in a Cu container in the preparation of the composite billet, or to repeat the procedure of composite billet formation in many times. Therefore, a good processability is desired, and the above-mentioned processes have a limit in this point. Further, increase in the number of filaments means that the distance between the superconducting filaments in the fine multifilamentary wire becomes shorter than in a conventional one. As a result, a physical coupling of filaments or a superconductive coupling owing to proximity effect will occur in a part or most part of the superconducting filaments to increase alternate current loss, typically hysteresis loss, resulting in deteriorating characteristics. Therefore, if a simple method other than stuffing single core wires into a Cu tube could be adopted in the preparation of the composite billet, not only simplification and cost reduction of the preparation process would be achieved, but also the superconducting characteristics would be improved.
An improved process for preparing a composite billet is proposed in Japanese Patent Publication Kokoku No. 54-22758 and U.S. Pat. No. 4,973,365. In the process proposed in the Japanese publication, a plurality of copper blocks each having a plurality of holes drilled in the longitudinal direction are stacked, superconducting material rods are inserted into the holes, copper covers are applied to both ends of the stacked blocks, and the periphery of the stacked blocks and covers are welded at their contacting portions in a vacuum by electron beams to form a billet to be extruded. U.S. Pat. No. 4,973,365 discloses a process for preparing a billet for extrusion by drilling a plurality of holes in a copper rod covered with a copper stabilizing layer and an Sn diffusion barrier layer and inserting Nb rods into the holes.
However, the former process has the problems that electron beam welding must be conducted in a vacuum in many times, thus the process steps become complicated to increase the preparation cost, and that the depth of weld penetration at the contact surface between the respective copper blocks is only 2 mm, so breaking of wire frequently occurs at the time of the sectional area reduction processing conducted thereafter. Also, the latter process has the problems that in practice it is difficult to drill a multiplicity of holes in a copper rod and it is impossible to secure a copper rod length of more than several meters.
It is an object of the present invention to solve the above-mentioned problems and to provide a process for preparing a superconducting wire having improved superconducting characteristics, which can achieve shortening of the preparation time and reduction of the preparation cost.
A further object of the present invention is to provide a process for preparing a superconducting wire which can be practiced with an improved processability or workability to improve the yield in a shortened period of time at a reduced preparation cost.
A still further object of the present invention is to provide a process for preparing a superconducting wire according to which alloy superconducting wires and compounds superconductive wires having improved superconducting characteristics can be easily prepared in a simple manner.
These and other objects of the present invention will become apparent from the description hereinafter.